Apply scale on all keyframes
You can use the scale function in the Graph editor the same way as in the 3D viewer. make sure that all the objects with keyframes you want to adjust are loaded in the Graph editor and visible, then use A to select all, then S,X and the amount you want to scale by (i.e. S,X,-,5 or S,X,1,.,2,5, etc. and press Enter.